Both Aggregators and Processors analyze metrics as they pass through Telegraf.

Processor plugins process metrics as they pass through and immediately emit
results based on the values they process. For example, this could be printing
all metrics or adding a tag to all metrics that pass through.

Aggregator plugins, on the other hand, are a bit more complicated. Aggregators
are typically for emitting new aggregate metrics, such as a running mean,
minimum, maximum, quantiles, or standard deviation. For this reason, all aggregator
plugins are configured with a period. The period is the size of the window
of metrics that each aggregate represents. In other words, the emitted
aggregate metric will be the aggregated value of the past period seconds.
Since many users will only care about their aggregates and not every single metric
gathered, there is also a drop_original argument, which tells Telegraf to only
emit the aggregates and not the original metrics.

NOTE That since aggregators only aggregate metrics within their period, that
historical data is not supported. In other words, if your metric timestamp is more
than now() - period in the past, it will not be aggregated. If this is a feature
that you need, please comment on this github issue